High energy gamma ray source



United States Patent 3,520,819 HIGH ENERGY GAMMA RAY SOURCE Albert E.Litherland, Toronto, and Thomas K. Alexander and Alan T. .lefls, DeepRiver, Ontario, Canada, as-

signors to Atomic Energy of Canada Limited, Ottawa,

Ontario, Canada, a corporation of Canada No Drawing. Filed Sept. '7,1967, Ser. No. 665,985

Int. Cl. C09k 3/00 US. Cl. 252301.1 4 Claims ABSTRACT OF THE DISCLOSUREA gamma ray source is described in which the capture of energetic alphaparticles by certain element nuclei result in an excited state of adaughter element of higher atomic number and which has a sufficientlylong life time in that state before decay by gamma radiation, so thatthe recoil velocity consequent upon the alpha particle absorption makesnegligible contribution to broadening of the gamma ray spectrum line.

The source of alpha particles must be energetic and preferably comprisesplutonium, curium, or americium and the element used for capture ispreferably boron or carbon 13.

The present invention relates to radiation sources and in particular toa mono-energetic source of 3.85 m.e.v. gamma rays.

Such a source is particularly useful for the testing of lithium driftedgermanium gamma ray detectors since an accurate calibration of thedetector at high energy levels is possible.

In general terms, the present invention provides a gamma ray sourceconsisting of a sintered intimate solid mixture of powdered boron 10 andan energetic alpha emitter such as plutonium. The plutonium may bepresent as plutonium oxide PuO Alpha particles emitted by the plutoniumreact with the nucleus of the boron in an alpha, proton, gamma reactionin which the boron is converted into carbon 13.

A source of mono-energetic 3.85 m.e.v. y rays, and lower energy 'y rays,has been produced by using B(Ot,p'y) C reactions occurring in anintimate mixture of B and PuO The -B(a,p) C reaction has a Q-value orenergy release of 4.07 m.e.v. and the first three excited states at3.09, 3.68 and 3.85 m.e.v. in C are populated when Pu a particles areused. The 3.85 m.e.v. level decays to the ground state (76% branch)giving rise to a 3.85 m.e.v. 'y ray. The other decay is to the 3.68m.e.v. level.

Although the excited C nuclei are produced with high recoil velocity(v/cE1% the Doppler broadening of the 'y rays from the 3.85 m.e.v. levelis almost completely attenuated since the lifetime (r =7.5+32ps) is longcompared to the slowing-down time of the recoils in the solid sourcematerial. Thus nearly all the decays from the 3.85 m.e.v. level occurfrom nuclei at rest.

In accordance with the prior art, a mixture of plutonium and Be is knownto produce a source of fast neutrons and high energy 'y rays by theBe(a,n) C reaction. This known source emits 4.43 m.e.v. 'y rays fromcarbon 12, however, these 'y rays are Doppler broadened to a width of-1% because the a particle direction is undefined and the recoilingcarbon 12 ions have not lost their high recoil velocity before they emitthe 'y rays. The large Doppler broadening of a Pu Be source makes itunsuitable for testing lithium drifted germanium detectors.

In contrast, the 'y ray source of the present invention with its 3.85m.e.v. 'y ray observed with a 40 cm. Ge(Li) detector, has a width of 5.8k.e.v. (full width at half maximum) which is presumably the systemresolution.

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In addition, the 24% branch to the 3.68 m.e.v. level is interestingsince the 3.68 m.e.v. 'y rays following the 170 k.e.v. cascade are alsomono-energetic. This gives rise to a sharp component and a Dopplerbroadened component due to direct feeding of the 3.68 m.e.v. level.

The Pu B 3.85 m.e.v. 'y ray source of the present invention was preparedin the following manner:

EXAMPLE I 10 g. of B powder, 3 g. of PuO powder (batch FP- 376, -200mesh, calcined at 800 C.), and mg. of Sterotex binding agent (i.e. 1%)were ground together in a new tool steel vial for 1 hour. The mixedpowders were then pressed in a 0.90 inch diameter die at 10 kp. s.i. andsintered in a flowing argon atmosphere at temperatures up to 1500" C.,then cooled. The density of the sintered pellet was measured as 1.84g./cm. which was less than the theoretical maximum density.

EXAMPLE II The sintered pellet of Example I was ground in a vibratorymixer mill using five grinding cycles of 30 mins. each. The groundpowder was pressed in a 0.90 inch diameter die at a higher pressure of50 kp. s.i., and then sintered in flowing argon atmosphere at 1600 C.for 2 hours then cooled.

Finally one face of the sintered pellet was polished using 2 ,um.diamond. An cc autoradiograph was taken using cellulose nitrate through.00025" Pt foil.

Pellet data: Diameter 2.292 cm., length 1.679 cm., volume 6.927 cm.weight 13.16 g., measured density 1.90 g./cm. (theoretical density is-2.40 g./cm.

Activity measurements: 7 30.0 mr./h., B 0.3 Rad, Neutron counts 22.4)(10c.p.m.

These results were obtained with a sintered 10 g. B 3 g. Pu0 sourcefabricated in the plutonium laboratory at Chalk River, Canada. Thereappeared to be scope for development to raise the yield of C byimprovements in density, particle size and the possible substitution ofAm or Pu as the source of 0: particles. Several sources using Am as thea particle emitter were then fabricated and have proved satisfactory.

With the increased a particle energy available it is energeticallypossible to populate the 6.13 m.e.v. level in 0 using the C(oc,n) Oreaction in a similar type of source. The lifetime of the 6.13 m.e.v.level is 251-2 ps and decays by a 6.13 m.e.v. 'y ray making thisreaction suitable for producing a mono-energetic 6.13 m.e.v. 'y ray.

The main uses of the Pu B source of the present invention are thetesting of the energy resolution and performance of Ge(Li) detectors,energy calibrations up to 3.85 m.e.v. and calibrations of detectorefficiency at 3.85 m.e.v.

It is a considerable advantage to have a convenient source ofmono-energetic 'y rays at 3.85 m.e.v. to measure the energy resolutionof Ge (Li) detectors. Charge collection efliciency in the detector andgain instabilities in the electronics are both percentage elfects andtherefore as the energy of the 'y ray is high, the quality of the Ge(Li) detector and its associated electronics are more critically tested.

The 3.85 m.e.v. full energy peak can be easily calibrated with precisionusing existing sources since the E.,2m c peak lies 1022 k.e.v. lower inenergy. At present, the most accurate value of the energy of the 'y rayis 3854i1 k.e.v. The 3.85 m.e.v. line can then be used as an energycalibration in experiments involving high energy 'y rays.

Since the source of 3.85 m.e.v. 7 rays is long lived, the relativeintensity of the 'y rays from the source remains fixed. Once theintensities are calibrated for a fixed geometry, the source can be usedto measure the efiiciency of Ge(Li) detectors; the fact that the 3.85m.e.v. line is sharp increases the accuracy with which this can be done.

Previously, reactions induced by beams of particles from acceleratorshad to be used for the test and calibration procedures. The use of a PuB source is more convenient and less expensive. It will be understoodthat the Pu B source need not be sinterized, but this ensures durabilitystrength and maintenance of size and shape.

We claim:

1. A gamma ray source for emitting essentially monoenergetic gamma rayscomprising an intimate mixture of a substance selected from the groupconsisting of boron 10 and carbon 13; and an emitter of c particlesselected from the group consisting of plutonium 239, plutonium 238,curium, and americium 241, and oxides thereof, said substance beingcombined with said emitter of 0: particles in the ratio of about :3.

2. A gamma ray source comprising an intimate solid mixture of boron 10and an energetic rx emitter selected from the group consisting ofplutonium 239, plutonium 238, curium, and americium 241 and oxidesthereof, said boron 10 being present in the ratio of about 10' partsboron 10 to 3 parts a emitter.

References Cited UNITED STATES PATENTS 3,320,176 5/1967 Davis 252l.13,360,477 12/1967 Acree et al. 25230l.1 3,361,857 1/1968 Rose 25230'1.1X

OTHER REFERENCES Atomic Energy of Canada Limited, Technical BulletinNS-l, Neutron Sources and Their Characteristics, July 21, 1961, pp. 1-7.

CARL D. QUARFORTH, Primary Examiner M. J. SCOLNICK, Assistant Examiner

